Pressure sensors have been placed in various locations relative to the cylinder chambers of internal combustion engines to monitor the pressures in the chambers during operation. For example, a shielded piezo-electric pressure sensor probe has been installed in a hole bored in the cylinder head for the purpose of holding the pressure sensor probe.
Sensors also have been integrated into spark plugs as an alternative to drilling holes in the cylinder head. For example, Lindner, U.S. Pat. No. 4,489,596, describes a spark plug arrangement having plates positioned within the spark plug. The plates include different types of sensors, such as a pressure sensor, a temperature sensor, and a gas composition sensor. Similarly, Steinke, U.S. Pat. No. 4,969,353, describes a spark plug having a bore extending through an outer portion of the spark plug so that a pressure sensor may be located within the bore. The central electrode is positioned nonconcentrically in the outer portion to provide space for accommodating the sensors.
Wlodarczyk, PCT Application No. PCT/US96/04637, describes a spark plug having a bore extending through an outer portion of the spark plug and containing a fiber optic diaphragm sensor. The diaphragm sensor may be exposed directly to the combustion gases within the engine cylinder.